A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g., comprising part of, one or several dies) on a substrate (e.g., a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
It is well-known in the art of lithography that the image of a mask pattern can be improved, and process windows enlarged, by appropriate choice of the angles at which the mask pattern is illuminated. In an apparatus having a Koehler illumination arrangement, the angular distribution of radiation illuminating the mask is determined by the intensity distribution in a pupil plane of the illumination system, which can be regarded as a secondary source. Illumination modes are commonly described by reference to the shape of the intensity distribution in the pupil plane. Conventional illumination, i.e., even illumination from all angles from 0 to a certain maximum angle, requires a uniform disk-shaped intensity distribution in the pupil plane. Other commonly-used intensity distributions are: annular, in which the intensity distribution in the pupil plane is in the shape of an annulus; dipole illumination, in which there are two poles in the pupil plane; and quadrupole illumination, in which there are four poles in the pupil plane. To create these illumination schemes, various methods have been proposed. For example, a zoom-axicon, that is a combination of a zoom lens and an axicon, can be used to create annular illumination with controllable inner and outer radii of the annulus. To create dipole and quadrupole type illumination modes, it has been proposed to use spatial filters, that is opaque plates with apertures where the poles are desired as well as arrangements using moveable bundles of optical fibers. Using spatial filters may be undesirable because the resulting loss of radiation reduces the throughput of the apparatus and hence increases its cost of ownership. Arrangements with bundles of optical fibers may be complex and inflexible. It has therefore been proposed to use a diffractive optical element (DOE) to form the desired intensity distribution in the pupil plane. The diffractive optical elements are made by etching different patterns into different parts of the surface of a quartz or CaF2 substrate.
A known type of radiation which is used within lithographic apparatus is deep ultraviolet (DUV) radiation. DUV radiation may have a wavelength of between about 100 nm and 300 nm, e.g., about 248 nm, about 193 nm, about 157 nm or about 126 nm. The choice of materials from which lenses useable with DUV radiation can be made is quite limited and even the best materials have significant coefficients of absorption of this radiation. This means that the lenses in the projection system absorb energy during exposures and heat up, leading to changes in their shape, separation and refractive index which introduce aberrations into the projected image. Therefore, many lens systems are provided with one or more actuated lens elements whose shape, position and/or orientation in one or more degrees of freedom can be adjusted during or between exposures to compensate for lens heating effects. Similar problems may occur within lithographic apparatus which has a generally reflective optical system, for example a lithographic apparatus which uses extreme ultra violet (EUV) radiation. Within such systems, the radiation of the lithographic apparatus may be absorbed by reflectors (such a mirrors), which may cause them to heat up and deform, thereby reducing the imaging performance of the lithographic apparatus.
If an illumination mode, such as dipole, in which the energy of the beam is strongly localized in a pupil plane of the illumination system is used, then the energy of the beam will also be strongly localized in and near the pupil plane(s) of the projection system. Lens heating effects are more severe when such localized illumination modes are used because the temperature gradients in the lens elements affected are greater, leading to localized changes in shape and/or refractive index which cause large phase gradients in the beam. These effects are often not correctable by existing actuated lens elements. Similar effects can be caused by the use of a slit-shaped illumination field, as is common in a scanning lithographic apparatus.